The easiest way to check if your house is encapsulating gases is to monitor the carbon dioxide (CO2) levels. This one gas is used as a marker for air quality, is the easiest to monitor, and provides a more accurate air quality reading.
It is a good trend indicator because it changes in measurable amounts as the air quality gets worse. All other gasses in the house follow the same trend as the CO2. Most commercial air quality controls are based on CO2 levels.
It is the easiest to monitor simply because as the air quality gets worse, the levels increase. Outside air has an average of 350 - 450 ppm of CO2. Inside, the levels raise to values of 550 - 700 ppm. In office buildings, the HVAC systems will keep the levels below 800 ppm.
Monitoring the CO2 levels is relatively inexpensive and more accurate compared to monitoring any of the other gases that are in your home such as carbon monoxide (CO) and TVOC. It is more accurate in that it does not change based on individual problems.